Population-level variation in the expression of heterostyly in three species of Rubiaceae: Does reciprocal placement of anthers and stigmas characterize heterostyly?

Heterostyly (i.e., reciprocal placement of anthers and stigmas between two or three floral morphs) is hypothesized to enhance outcrossing and reduce s elfing. However, few studies have documented reciprocity among individual p lants. instead, mean anther and stigma heights for floral morphs are usuall y reported, masking interindividual variation. We measured eight floral dim ensions for individuals in five populations of three heterostylous Rubiacea e. The three methods used to quantify reciprocity yielded different conclus ions regarding the degree to which populations conformed to expectations fo r heterostylous plants. Only Psychotria poeppigiana had stigma and, to a le sser degree, anther heights in discrete classes. Variation among plants of Bouvardia ternifolia and Psychotria chiapensis yielded a continuum of anthe r and stigma heights across populations. Comparison of distances between st igma and anthers indicated that only flowers of B. ternifolia had, as expec ted, a constant value for this distance. Finally, regression relationships between anther and stigma heights and corolla length showed that only in on e population each of B. ternifolia and P. poeppigiana, and in P. chiapensis , was distance between anthers and stigmas the same across the range of cor olla sizes for both floral morphs. Variation among these species in express ion of heterostyly was not clearly linked to phylogenetic relationship or p ollinator syndromes. Two approach herkogamous (AH) species were studied for comparison. Flowers of Psychotria brachiata were consistently AH, but flow ers of P. pittieri were highly variable. Determining fitness consequences o f population-level variation in sexual systems requires studies linking flo ral morphology to pollinator behavior and pollen transfer.